S.L.I

For SLI to work you need two Graphics Cards that match and say a DDR3 SLI Bridge, and a Motherboard that supports SLI "eg" two PCI-E slots. SLI (Scalable Link Interface) is a brand name for a multi-GPU solution developed by nVidia (thats two nVidia Graphics Cards running in sync on one Motherboard) this configuration when gaming works with (profiles) nVidia has created an extensive set of over 500 custom application profiles which enable SLI technology automatically and optimise scaling performance, you can also submit a request to have this profile for a game not in the list added to the next driver release. Picture and more to read in the link below >

Click on image to enlarge

Link > nVidia SLI Zone

Crossfire

ATI - Now ATI / AMD Crossfire But i will just call them ATI
This Graphics Card system requires a Crossfire-compliant Motherboard with a pair of PCI Express (PCIe) graphics cards, which can be enabled via either hardware or software. Radeon x800s, x850s, x1800s and x1900s come in a "Crossfire Edition"' that has "master" capability built into the hardware. So you need a Master card, and pair it with a normal card from the same series. Radeon x1300s and x1600s have no "Crossfire Edition" but are enabled via software. ATI currently has not created the infrastructure to allow FireGL cards to be set up in a Crossfire configuration. Another point to note is that the "slave" graphics card needs to be from the same family as the "master", regardless of whether the master is designated by the hardware or by software, ATI`s Catalyst.
With the release of the Radeon X1950 Pro GPU, ATI revised Crossfire's connection infrastructure to eliminate the need for past Y-Dongle/Master card and slave card configurations for Crossfire to operate. ATI's Crossfire connector is now a ribbon like connector attached to the top of each Graphics Card, similar to nVidia's SLI bridge, but different in physical and logical natures.
nVidia and ATI are locked in a Graphics Card War, nVidia consider ATI to be a threat and vice versa, each camp produce some good Cards more on Crossfire in the links below >

Link > Crossfire Physics

Link > Crossfire How it works

Note A single new high end Graphics Card is capable of playing about any game on the market

The ATI Crossfire is now integrated in to many Motherboards now so no need for the bridge also with the emergence of twin chip graphics cards crossfire is in a single card, if two are installed crossfire X2 is achieved, nVidia are ATM still a bit behind in this field.

Cat

I usually consider SLI/Crossfire to be a waste of money, but i have to admit that the 9600GT looks to be a card worth running in SLI. that, and with the price drops on the 3870s, Crossfire looks good now, too.

Rumours of quad chip GPUs are around "but its the same old story whatever the big guys (nVidia / ATI) crank out software will be playing catch up", not found a game that my ATI Radeon HD 3870 X2 "Crossfire in one" cant play, Crysis is graphics intense but about all the high cards can walk it at max settings

Those dual GFX chip cards are a gimmick as far as I'm concerned. I would rather run a single non-Frankenstein GPU and have no driver issues, even if I can't run whatever at the highest settings. I never use the highest setting when I game, anyway... I use settings that give me a visual advantage.

^ Each to there own